1. Field of the Invention
This invention relates to adaptive optics systems for communicating with a distant location through a distorting atmosphere, and more particularly to large area adaptive optics systems in which the distortions in a received communications beam are detected and used to apply compensatory distortions to a transmission beam.
2. Description of the Related Art
A problem has long existed in communicating through the atmosphere with distant objects via optical beams, such as visible light, radar or infrared (IR). The earth's atmosphere is a distorting medium which produces phase shifts in the beam, causing the received beam to differ from the beam as originally transmitted. Normal atmospheric turbulence causes the beam distortion to continuously change, and also to vary from point to point within the beam. Thus, there are problems of both temporal and spatial distortions.
FIG. 1 illustrates the general approach of an adaptive optics system, the purpose of which is to compensate for these distortions. Assume that communication is desired through the atmosphere with a remote target, such as satellite 2. It might be desired to have the satellite reflect a transmitted beam onto a designated target area. The atmospheric turbulence through which the communications must pass is indicated by wavy lines 4. A reference or probe beam 6 having known phase characteristics is transmitted through the atmosphere to the receiving location, where it is analyzed by a distortion sensor 8. Here the actual phase characteristics of the reference beam after passage through the distorting atmosphere are detected in a conventional wavefront error sensor. As a result of this detection, the magnitude and nature of the atmospheric distortion can be determined. This information is applied to a negative distortion device 10, which sets up a distortion that is complimentary to that produced by the atmosphere. A transmission beam 12 is applied to the negative distortion device 10 and picks up a spatial phase distortion which is the opposite of that which it will encounter during transit through the atmosphere to the satellite 2. The result ideally is that the complementary distortions imposed upon the transmission beam by the negative distortion device 10 and by the atmosphere will cancel each other, permitting the satellite to receive the transmission beam in substantially undistorted form.
The negative distortion device has typically been implemented as a spatially deformable mirror, or as a matrix of segmented mirror segments. In one class of mirror the wavefront is controlled by an electrically activated array of pistons (such as a 30.times.30 array) to move the various portions of the mirror in or out. While a general distortion compensation can be achieved by this approach, the deformable and segmented mirrors suffer from poor resolution and limited size, and are structurally complex. Also, it is difficult to implement them with a response time fast enough to keep up with the normal changes in atmospheric distortion, which typically varies due to turbulence at about 1KHz. A review article which describes the adaptive optics concept and implementations thereof was written by John Hardy, "Active Optics: A New Technology for the Control of Light", Proceedings of the IEEE, Vol. 66, No. 6, June, 1979, pages 651-97.